Abstract

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) uses nanocrystalline diamond foils to strip the electrons from an H- beam in order to create protons which are used later in the spallation process. Many of the foils in use at the SNS were prepared via plasma-assisted Chemical Vapor Deposition (PACVD), which uses a high argon gas content in the reactant gases coupled with a microwave plasma to produce nanocrystalline diamond on a silicon wafer. With foil flutter and curling of the diamond foils as current issues at SNS, different growth methods are under test to engineer the diamond foils for the desired mechanical and electrical properties needed for extended life under the intense particle beam. We characterized two different methods of nanocrystalline diamond growth, including the well-researched PACVD method and a hot filament CVD (HFCVD) method conducted by Blue Wave Semiconductors, Inc. Using a scanning electron microscope (SEM), weight bearing tests and alpha ranging we were able to characterize the uniformity and similarities of the nanocrystalline material from the two methods. The results of the alpha ranging and stiffness testing of the free standing diamond indicated that the two methods yielded similar structures. However, our results also indicate that the HFCVD method produces greater size variations of the nanocrystallites compared to the PACVD, based on SEM analyses. The HFCVD growth method is a candidate for producing electron stripping foils because it may ultimately lead to better foil thickness uniformity. It may also result in less stress in the free-standing diamond and thus be less likely to curl under the heat of the intense particle beam. Preliminary neutron activation analysis indicates a high concentration of detrimental filament tungsten is incorporated into the HFCVD foils, while our preliminary conductivity measurements indicate that the HFCVD foils have high impedance which limits the amount of current that they can dissipate. These two properties are a detriment to the use of HFCVD foils at the SNS.